A fuel cell system has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One type of fuel cell system employs a proton exchange membrane (PEM) to catalytically react a hydrogen fuel and an oxidant to generate electricity. Typically, the fuel cell system has more than one fuel cell including an anode and a cathode with the PEM therebetween. The anode receives hydrogen gas and the cathode receives oxygen, typically from air. The hydrogen gas is ionized in the anode to generate free hydrogen ions and electrons. The hydrogen ions pass through the PEM to the cathode. The hydrogen ions react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the PEM, and are instead directed through an electric load, such as a vehicle, to perform work before being sent to the cathode. Many fuels cells may be combined in a fuel cell stack within the fuel cell system to generate a desired quantity of power.
The fuel cell power system can include a reformer or processor that converts a liquid fuel, such as alcohols (e.g., methanol or ethanol), hydrocarbons (e.g., gasoline), and/or mixtures thereof, such as blends of ethanol/methanol and gasoline, to the hydrogen gas for the fuel cell stack. More typically, the hydrogen gas employed as a fuel in the fuel cell system is processed separately from the vehicle and stored. The hydrogen gas is transferred to a high pressure vessel or container on the vehicle to supply the desired hydrogen gas to the onboard fuel cell stack as needed.
High pressure vessels are typically classified into one of four types: a Type I vessel having an all-metal construction; a Type II having a metal-lined construction with a fiber hoop wrap for reinforcement; a Type III having a metal-lined construction with a complete fiber reinforcement wrap; and a Type IV having a plastic-lined construction with a complete fiber reinforcement wrap. As disclosed by Immel in U.S. Pat. No. 6,742,554, hereby incorporated herein by reference in its entirety, the Type IV pressure vessel contemplated in the industry for storage of hydrogen gas is generally cylindrical in shape to provide the desired integrity, and includes an outer structural wall and an inner liner defining a container chamber therein. A Type V vessel having a liner-less composite construction has also been contemplated in the art.
High pressure vessels containing a compressed hydrogen gas must have a desired mechanical stability and integrity that militates against a rupture or bursting of the pressure vessel from the internal pressure. It is also typically desirable to make the pressure vessels on vehicles lightweight so as not to significantly affect the weight requirements of the vehicle. The current trend in the industry is to employ the Type IV pressure vessel for storing the compressed hydrogen gas on the vehicle.
Known high pressure vessels include at least one thermally activated safety valve or pressure relief device (PRD). The PRD is located at a boss at an end of the high pressure vessel that houses various valves, pressure regulators, piping connectors, excess flow limiters, etc. for allowing the pressure vessel to be filled with the compressed hydrogen gas. The PRD may also be located at another opening in the pressure vessel, though the PRD generally is disposed at one or both ends of the pressure vessel. The PRD is useful when the pressure vessel is exposed to high temperatures. More than one PRD may be used where high temperatures might occur at a localized area apart from the location of the single PRD.
It is known that a localized heat source not adjacent an end of the pressure vessel with the PRD may not be detected by the PRD due to the low thermal conductivity of the composite materials forming the pressure vessel. A state of the art solution to this problem is to employ a heat pipe to transfer heat from the area of the pressure vessel adjacent the localized heat source. A heat pipe does not cover the entire surface of the pressure vessel, however. Other known designs use an additional insulating layer to reduce heat flux into the pressure vessel. The additional insulating layer only delays the rupture of the pressure vessel, however, and is therefore undesirable.
Pressure vessel systems with heat conducting layers for transferring heat from anywhere on the pressure vessel to the PRD are also described in U.S. Appn. Pub. No. 2008/0066805 to Winter et al., hereby incorporated herein by reference in its entirety. The known heat conducting layers include heat conducting mesh wrapped around and outside of the pressure vessel, and heat conducting strips connected to the PRD that extend along the pressure vessel. Heat conducting sheets, foils, and layers wrapped all around an outside of the pressure vessel are also known.
There is a continuing need for a pressure vessel that is effective in transporting heat from localized heat sources to the PRD, and facilitates the use of a single PRD instead of multiple PRDs. Desirably, the pressure vessel is provided with an additional protective layer that carries a portion of the load across the pressure vessel, militates against the need for an additional stone shield, and is more cost effective than present pressure vessel designs.